Superscalar Phase Boundaries Derived Multiple Active Sites in SnO₂/Cu₆Sn₅/CuO for Tandem Electroreduction of CO₂ to Formic Acid

Abstract The electrocatalytic CO 2 reduction reaction (CO 2 RR) to fuels driven by electrocatalysts is a viable strategy for efficient utilization of emitted CO 2 . CO 2 RR involves multiple‐steps, including adsorption, activation, hydrogenation, etc. At present, copper‐tin alloy catalysts have shown the capability to reduce CO 2 to formic acid or formate. However, their poor adsorption and activation capacities for CO 2 molecules, as well as the sluggish kinetics in *H supply restrict the proton‐coupled electron transfer processes in the electrocatalytic CO 2 RR to produce formic acid. In order to solve the above problems, the ultra‐small SnO 2 /Cu 6 Sn 5 /CuO nanocatalysts with superscalar phase boundaries are fabricated by laser sputtering. The introduction of SnO 2 enhances the adsorption and activation of CO 2 , while CuO promotes H 2 O decomposition and provides abundant *H intermediates, resulting in tandem catalytic sites on the SnO 2 /Cu 6 Sn 5 /CuO composite catalysts and thus leading to excellent CO 2 RR activity and high selectivity to formic acid. The Faradic efficiency of formic acid (FE HCOOH ) at the SnO 2 /Cu 6 Sn 5 /CuO electrode reaches 90.13% along with a high current density of 25.2 mA cm −2 at −0.95 V versus reversible hydrogen electrode. The role of the multiphase boundaries constructed by introduction of oxides is confirmed by in situ infrared spectroscopy and kinetic isotope effects experiments, which is consistent with the design concept.